PWM/PFM STEP-DOWN COMBINATION
REGULATOR/CONTROLLER
TC120
PWM/PFM STEP-DOWN COMBINATION REGULATOR/CONTROLLER
FEATURES
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Internal Switching Transistor Supports 600 mA
Output Current
External Switching Transistor Control for Output
Currents of 2A+
300 KHz Oscillator Frequency Supports Small
Inductor Size
Short Circuit Protection
Built-In Undervoltage Lockout
High (95%, Typ) Efficiency
Automatic Switchover to Current-Saving PFM
Mode at Low Output Loads
Automatic Output Capacitor Discharge While in
Shutdown
Programmable Soft-Start Time
Power-Saving Shutdown Mode
Small 8-Pin SOP Package
GENERAL DESCRIPTION
TC120 is a 300 KHz PFM/PWM step-down (Buck) DC/
DC regulator/controller combination for use in systems
operating from two or more cells, or in line-powered applica-
tions. It uses PWM as the primary modulation scheme, but
automatically converts to PFM at low output loads for greater
efficiency. It requires only an external inductor, Schottky
diode, and two capacitors to implement a step-down con-
verter having a maximum output current of 600 mA (V
IN
=5V,
V
OUT
= 3.3V). An external switching transistor (P-channel
MOSFET) can be added to increase output current capabil-
ity to support output loads of 2A or more.
The TC120 consumes only 39
µA
(max) of supply
current (V
OUT
= 3.3V) and can be placed in shutdown mode
by bringing the shutdown input (SHDN) low. During shut-
down, the regulator is disabled, supply current is reduced to
2.5
µA(max),
and V
OUT
is internally pulled to ground, dis-
charging the output capacitor. Normal operation resumes
when SHDN is brought high. Other features include a built-
in undervoltage lockout (UVLO), an externally program-
mable soft start time, and output short circuit protection. The
TC120 operates from a maximum input voltage of 10V and
is available in a low-profile 8-Pin SOP package.
TYPICAL APPLICATIONS
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Portable Test Equipment
Local Logic Supplies
Portable Audio Systems
Portable Scanners
Palmtops
Electronic Organizers
ORDERING INFORMATION
Part No.
Output
Voltage (V)
3.0
3.3
5.0
Package
TYPICAL APPLICATION
TC120303EHA
TC120333EHA
TC120503EHA
Temperature
Range
V
IN
= 5V
L1
22
µH
V
OUT
= 3V
8-Pin SOP – 40°C to +85°C
8-Pin SOP – 40°C to +85°C
8-Pin SOP – 40°C to +85°C
V
IN
47
µF/16V
Tantalum
L
X
IN5817
C
OUT
47
µF/10V
Tantalum
EXTW
EXT
PIN CONFIGURATION
TC120303
CPC
GND
8-Pin SOP
V
IN
1
EXTW 2
CPC 3
SHDN/SS 4
TC120
8
L
X
SHDN/SS
C
SS
4.7 nF
SENSE
7 EXT
6
5
GND
SENSE
5V to 3.0V, 500 mA Step-Down Converter
TC120-1 7/27/99
1
TelCom Semiconductor reserves the right to make changes in the circuitry and specifications of its devices.
PWM/PFM STEP-DOWN COMBINATION
REGULATOR/CONTROLLER
TC120
ABSOLUTE MAXIMUM RATINGS*
Power Supply Voltage (V
IN
) ....................... –0.3V to +12V
*Static-sensitive device. Unused devices must be stored in conductive
Voltage on V
OUT
Pin ................................... –0.3V to +12V
material. Protect devices from static discharge and static fields. Stresses
Voltage on LX, Boost Pins ...... (V
IN
– 12V) to (V
IN
+ 0.3V)
above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. These are stress ratings only and functional
Voltage on EXT1, EXT2, SHDN Pins
operation of the device at these or any other conditions above those
................................................ (–0.3V) to (V
IN
+ 0.3V)
indicated in the operational sections of the specifications is not implied.
L
X
Pin Current .......................................................700 mA
Exposure to Absolute Maximum Rating Conditions for extended periods
EXT1, EXT2 Pin Current .......................................±50 mA
may affect device reliability.
Continuous Power Dissipation .............................300 mW
ELECTRICAL CHARACTERISTICS:
(Test Circuit of Figure 1, T
A
= 25°C, V
IN
=V
R
x 1.2, Note 1, unless other-
wise noted)
Symbol
V
OUT
Operating Temperature (T
C
) ...................... –40°C to 85°C
Storage Temperature (T
STG
) .................... –40°C to 150°C
Parameter
Output Voltage
Test Conditions
V
R
= 3.0, I
OUT
=120mA (Note 1)
V
R
= 3.3, I
OUT
=132mA
V
R
= 5.0, I
OUT
=200mA
V
OUT
= 3.0V
V
OUT
= 3.3V
V
OUT
= 5.0V
V
OUT
= 3.0V
V
OUT
= 3.3V
V
OUT
= 5.0V
Min
Typ
Max
Unit
V
V
R
x 0.975 V
R
±
0.5% V
R
x 1.025
V
IN
I
OUT(MAX)
Input Voltage
Maximum Output Current
1.8
500
600
600
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
52
55
71
1.5
—
1.5
0.69
0.64
0.44
38
35
24
10
—
—
—
82
86
110
2.5
2
2.5
0.94
0.85
0.58
52
47
32
V
mA
I
IN
I
SHDN
I
LX
R
DSON(LX)
Supply Current
V
IN
= V
R
x 1.05,
No Load
µA
µA
µA
Ω
Shutdown Supply Current
LX Pin Leakage Current
LX Pin ON Resistance
R
EXTH
EXT1, EXT2
On Resistance to V
IN
R
EXTL
EXT1, EXT2
On Resistance to GND
f
OSC
D
PWM
D
PFM
η
V
UVLO
V
IH
V
IL
t
PRO
t
SS
Notes:
TC120-1 7/27/99
Oscillator Frequency
Maximum PWM Duty Cycle
PFM Duty Cycle
Efficiency
Minimum Operating Voltage
(Note 2),
No Load, SHDN = 0V
Measured at EXT 1 Pin (Note 2)
No Load, SHDN = 0V
V
OUT
= V
R
x 0.9 (Note 2) V
OUT
= 3.0V
V
LX
= V
IN
– 0.2V, 10Ω
V
OUT
= 3.3V
Resistor from L
X
to V
IN
, V
OUT
= 5.0V
SHDN = V
IN
(Note 2);
V
OUT
= 3.0V
SHDN = V
IH
; EXT1 and V
OUT
= 3.3V
EXT 2 connected to
V
OUT
= 5.0V
200Ω load, V
EXT1
=
V
EXT2
= (V
IN
– 0.4V);
V
OUT
= V
IN
(Note 2);
V
OUT
= 3.0V
SHDN = V
IH
; EXT1 and V
OUT
= 3.3V
EXT2 pulled up through V
OUT
= 5.0V
a series resistance of
200Ω to a voltage such
that VEXT1, 2 = 0.4V
Measured at EXT1 pin, V
IN
= V
OUT
+0.3V,
I
OUT
= 20 mA, (Note 3)
No Load
V
IN
> V
R
x 1.2
V
OUT
= V
R
x 0.9 (Note 2), SHDN = V
IN
Measured with internal transistor in OFF
state and V
IN
falling.
(Note 2), V
OUT
= 0V
(Note 2), V
OUT
= 0V
(Note 2) Time from V
OUT
= 0V to
SHDN = V
IL
Ω
—
—
—
31
29
20
41
37
26
Ω
255
–
15
—
0.9
300
—
25
95
—
345
100
35
—
1.8
KHz
%
%
%
V
SHDN Input Logic High
Threshold Voltage
SHDN Input Logic Low
Threshold Voltage
Short Circuit Protection
Response Time
Soft Start Time
0.65
—
3
6
—
—
5
10
—
0.2
8
16
V
V
msec
msec
1. V
R
is the factory-programmed output voltage setting.
2. No external components connected, except C
SS
.
3. While operating in PWM Mode.
2
PWM/PFM STEP-DOWN COMBINATION
REGULATOR/CONTROLLER
TC120
PIN DESCRIPTION
Pin Number
1
2
Name
V
IN
EXTW
Description
Unregulated Supply Input.
Extended External Switching Transistor Drive Output. This output follows the timing on the
EXT output with an additional 100 nsec blanking time on both the leading and trailing edges.
That is, this output transitions from high-to-low 100 nsec
prior
to the same transition on EXT;
and transitions low-to-high 100 nsec
after
the same transition on EXT; resulting in a longer
external switch ON time. (See
Operation as a Regulator Controller
for more information).
Charge Pump Capacitor Input. An inverting charge pump is formed by attaching a capacitor
and diode to this input (please see
Improving High Load Efficiency In Regulator Operating
Mode
section).
Shutdown and Soft-Start Control Input. A soft start capacitor of 100 pF (min)
must
be
connected to this input. The soft start capacitor is charged by an internal 1µA current source
that gently ramps the TC120 into service. Shutdown control is best implemented with an
external open collector (or open drain) switch. The TC120 enters shutdown when this input is
low. During shutdown, the regulator is disabled, and supply current is reduced to less than
2.5
µA.
Normal operation is restored when this input is open-circuited, and allowed to float
high. Please see
SHDN/SS
section for details.
Voltage Sense Input. This input must be connected to the output voltage node at the
physical location that requires the tightest voltage regulation.
Ground Terminal.
External Switching Transistor Drive Output. This output connects directly to the gate of an
external P-channel MOSFET for applications requiring output currents greater than 600 mA.
The timing of this output exactly matches that of the gate drive for the internal P-channel
transistor. This output can drive a maximum capacitance of 1000 pF. (See
Operation as a
Regulator Controller
for more information).
Inductor Terminal. This pin is connected to the drain of the internal P-channel switching
transistor. If the TC120 is operated as a regulator (i.e. using the internal switch); the inductor
must be connected between this pin and the SENSE pin.
3
CPC
4
SHDN/SS
5
6
7
SENSE
GND
EXT
8
L
X
V
IN
L1
22
µH
V
OUT
V
IN
47
µF/10V
Tantalum
L
X
IN5817
C
OUT
47
µF/10V
Tantalum
EXTW
EXT
TC120xx03
CPC
GND
SHDN/SS
C
SS
4.7 nF
SENSE
Figure 1. Test Circuit
currents to a maximum of 600 mA with operating efficiencies
above 85%. (Efficiencies at high loads can be further im-
proved by using the on-board charge pump circuit to pull the
gate of the internal switching transistor below ground for the
lowest possible ON resistance. For more information, see
Improving High Load Efficiency in Regulator Operating
Mode
section).
Higher output currents are achieved by operating the
TC120 with an external P-Channel switching transistor
(Controller mode). In this operating configuration, the maxi-
mum output current is determined primarily by the ON
resistance of the P-Channel switch and the series resis-
tance of the inductor.
DETAILED DESCRIPTION
The TC120 can be operated as an integrated step-down
regulator (using the internal switching transistor); or as a
step-down regulator controller (using an external switching
transistor). When operating as an integrated regulator, the
only required external components are a Shottky diode,
inductor and an output capacitor. Operating in this configu-
ration, the TC120 is capable of supporting output load
TC120-1 7/27/99
Inductor Selection
Selecting the proper inductor value is a trade-off be-
tween physical size and power conversion requirements.
Lower value inductors cost less, but result in higher ripple
current and core losses. They are also more prone to
saturate since the coil current ramps faster and could
overshoot the desired peak value. This not only reduces
efficiency, but could also cause the current rating of the
3
PWM/PFM STEP-DOWN COMBINATION
REGULATOR/CONTROLLER
TC120
external components to be exceeded. Larger inductor val-
ues reduce both ripple current and core losses, but are
larger in physical size and tend to increase the start-up time
slightly. A 22
µH
inductor is the best overall compromise and
is recommended for use with the TC120. For highest effi-
ciency, use inductors with a low DC resistance (less than 20
mΩ). To minimize radiated noise, consider using a toroid,
pot core or shielded-bobbin inductor.
will require the addition of a clamping mechanism (such as
a Zener diode) to limit the voltage as described. While this
technique improves efficiency at high output loads, it is at the
expense of low load efficiency because energy is expended
charging and discharging the charge pump capacitor. This
technique is therefore not recommended for applications
that operate the TC120 at low output currents for extended
time periods. If unused, CPC must be grounded.
Input Bypass Capacitor
Using an input bypass capacitor reduces peak current
transients drawn from the input supply, and reduces the
switching noise generated by the regulator. The source
impedance of the input supply determines the size of the
capacitor that should be used.
Low Power Shutdown Mode/Soft Start Input
The SHDN/SS input acts as both the shutdown control
and the node for the external soft start capacitor, which is
charged by an internal 1
µA
current source. A value of 4700
pF (100 pF minimum) is recommended for the soft start
capacitor.
IMPORTANT: The soft start capacitor must be
connected between SHDN/SS and ground.
Failure to do this
may cause large overshoot voltages and/or large inrush
currents resulting in possible instability. The TC120 enters
a low power shutdown mode when SHDN/SS is brought low.
While in shutdown, the oscillator is disabled and the output
discharge switch is turned on, discharging the output ca-
pacitor. Figure 4 shows the recommended interface circuits
to the SHDN/SS input. As shown, the SHDN/SS input should
be controlled using an open collector (or open drain) device,
such that the SHDN/SS input is grounded for shutdown
mode, and open-circuited for normal operation (Figure 5a).
If a CMOS device is used to control shutdown (Figure 5b),
the value of R1 and C
SS
should be chosen such that the
voltage on SHDN/SS rises from ground to 0.65V in 1.5 msec
(Figure 6). If shutdown is not used, C
SS
must still be
connected as shown in Figures 5c and 5d. SHDN/SS may
be pulled up with a resistor (Figure 5c) as long as the values
of R
SS
and C
SS
provide the approximate charging
characteristic on power up shown in Figure 6. C
SS
only may
also be connected as shown in Figure 5d with C
SS
chosen
at 4700 pF (minimum 100 pF).
Output Capacitor
The effective series resistance of the output capacitor
directly affects the amplitude of the output voltage ripple.
(The product of the peak inductor current and the ESR
determines output ripple amplitude.) Therefore, a capacitor
with the lowest possible ESR should be selected. Smaller
capacitors are acceptable for light loads or in applications
where ripple is not a concern. A 47
µF
Tantalum capacitor is
recommended for most applications. The Sprague 595D
series of tantalum capacitors are amongst the smallest of all
low ESR surface mount capacitors available. Table 1 lists
suggested component numbers and manufacturers.
Catch Diode
The high operating frequency of the TC120 requires a
high-speed diode. Schottky diodes such as the MA737 or
1N5817 through 1N5823 (and the equivalent surface mount
versions) are recommended. Select a diode whose average
current rating is greater than the peak inductor current; and
whose voltage rating is higher than V
IN(MAX)
.
Undervoltage Lockout (UVLO)
Improving High Load Efficiency in Regulator
Operating Mode
If the TC120 is operated at high output loads most (or all)
of the time, efficiency can be improved with the addition of
two components. Ordinarily, the voltage swing on the gate
of the internal P-Channel transistor is from ground to V
IN
. By
adding a capacitor and diode as shown in Figure 3, an
inverting charge pump is formed, enabling the internal gate
voltage to swing from a negative voltage to +V
IN
. This
increased drive lowers the R
DS(ON)
of the internal transistor,
improving efficiency at high output currents. Care must be
taken to ensure the voltage measured between V
IN
and CPC
does not exceed an absolute value of 10V. While this is not
a problem at values of V
IN
at (or below) 5V, higher V
IN
values
TC120-1 7/27/99
The TC120 is disabled whenever V
IN
is below the
undervoltage lockout threshold. This threshold is equal to
the guaranteed minimum operating voltage for the TC120
(i.e. 2.2V). When UVLO is active, the TC120 is completely
disabled.
Short Circuit Protection
Upon detection of an output short circuit condition, the
TC120 reduces the PWM duty cycle to a minimum value
using its internal protection timer. The sequence of events is
as follows: when an output voltage decrease to near zero is
detected (as the result of an overload), the internal (5 msec)
protection timer is started. If the output voltage has not
4
PWM/PFM STEP-DOWN COMBINATION
REGULATOR/CONTROLLER
TC120
recovered to nominal value prior to the expiration of the
protection timer, the TC120 is momentarily shut down by
dedicated, internal circuitry. Immediately following this ac-
tion, the soft start sequence is engaged in an attempt to re-
start the TC120. If the output short circuit is removed, normal
operation is automatically restored. If the short circuit is still
present, the timed self-shutdown sequence described above
is repeated.
suppliers of external components recommended for use
with the TC120.
Board Layout Guidelines
As with all inductive switching regulators, the TC120
generates fast switching waveforms, which radiate noise.
Interconnecting lead lengths should be minimized to keep
stray capacitance, trace resistance and radiated noise as
low as possible. In addition, the GND pin, input bypass
capacitor and output filter capacitor ground leads should be
connected to a single point. The input capacitor should be
placed as close to power and ground pins of the TC120 as
possible. The length of the EXT trace must also be kept as
short as possible.
Operation as a Regulator Controller
External Switching Transistor Selection
EXT is a complimentary output with a maximum ON
resistances of 32Ω to V
DD
when high and 26Ω to ground
when low, at V
OUT
= 5V. It is designed to directly drive a P-
channel MOSFET (Figure 7). The P-channel MOSFET
selection is determined mainly by the on-resistance, gate-
source threshold and gate charge requirements. Also, the
drain-to-source and gate-to-source breakdown voltage rat-
ings must be greater than V
IN(MAX)
. The total gate charge
specification should be less than 100 nC for best efficiency.
The MOSFET must be capable of handling the required
peak inductor current, and should have a very low on-
resistance at that current. For example, a Si9430 MOSFET
has a drain-to-source rating of –20V, and a typical on-
resistance r
DS(ON)
of 0.07Ω at 2A, with V
GS
= –4.5V. (EXTW
(Figure 8) may be gated with external circuitry to add
blanking, or as an auxiliary timing signal.) Table A lists
V
IN
L1
V
OUT
V
IN
C
IN
L
X
D1
C
OUT
EXTW
EXT
TC120xx03
CPC
GND
SHDN/SS
C
SS
4.7 nF
SENSE
Figure 2. TC120 Typical Application
V
IN
≤
5V
V
IN
> 5V
C
P
2200 pF
Ceramic
V
IN
L
X
C
P
2200 pF
Ceramic
V
IN
L
X
EXTW
EXT
EXTW
EXT
TC120xx03
CPC
D
1
IN5817
TC120xx03
CPC
10V
Zener
Diode
D
1
IN5817
GND
GND
SHDN/SS
SENSE
SHDN/SS
SENSE
a) For V
IN
≤
5V
b) For V
IN
> 5V
Figure 3. TC120 with Added Components for Improved Efficiency at High Output Currents
TC120-1 7/27/99
5
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